1. Field of Endeavor
The invention, in its several embodiments, pertains generally to the measurement and control of ascorbate in liquid solutions and particularly to the monitoring and control of ascorbate based on refractivity, and/or the electrical conductivity, of a liquid solution sample and/or the monitoring and control of calcium ascorbate based on the concentration of calcium ions in the liquid solution.
2. State of Technology
Ascorbate is a salt, e.g., calcium ascorbate and sodium ascorbate, or other derivatives of ascorbic acid. Ascorbate compound solution is used as an anti-oxidant dip for the prevention of the browning of fruit and vegetable surfaces, and preserving the appearance, texture, crispness and color of fresh cut or minimally processed fruits and vegetables, such as apples. Proper management via continuous or continual monitoring and control of ascorbate concentration levels in the dip tank during processing enhances the effectiveness of the ascorbate as a preservative for fresh fruit and vegetables. Brine solutions containing ascorbic acid have been proposed in U.S. Pat. No. 4,883,679 to Sewón. Compositions of edible ingredients in solution and a dipping process for treating freshly cut surfaces of edible plants have been proposed in U.S. Pat. No. 4,988,522 to Warren. Methods of preserving fresh fruit have been proposed such as: (a) using calcium ascorbate according to U.S. Pat. No. 5,939,117 to Chen et al.; (b) using ascorbic acid according to U.S. Pat. No. 6,749,875 to Selleck; and (c) using calcium ions and ascorbate ions according to WO 94/12041 by Lidden.
Calcium ion concentrations in solutions can be measured based on the electrical properties of the solution, particularly by electrodes that are ion specific. Calcium ion-specific electrodes have been proposed for use in the measuring of calcium ions, e.g., as disclosed by U.S. Pat. No. 4,724,216 to Young et al., and by EP0304151 to Musacchio et al.
A refractometer is an instrument that measures the refractive index based on a principle that the refractive index of a solution increases in proportion to the concentration of the solute in the solution. As illustrated in FIG. 1, if the refractive index of air under atmospheric pressure is unity, when light enters medium x, the ratio of the sine of the incident angle, α, measured against the phase boundary to the sine of the refracting angle, β, is called the refractive index of the medium, x. Brix is related to the concentration of dissolved solids, e.g., sucrose, in a fluid and is related to the specific gravity of the liquid. Because the specific gravity of sucrose solutions is well known, it can also be measured by refractometers. Modern Brix meters are typically digital refractometers that calculate the Brix value based on a refractive index. A Brix percentage scale is recommended by the International Committee of Uniform Method of Sugar Analysis (ICUMSA). Brix meters are used in the food industry for measuring the approximate amount of sugars in fruits, vegetables, juices, wine, soft drinks and in the sugar manufacturing industry.
Electrical conductivity is used as a basis for assessing the progress of electrodialysis in the making of ascorbic acid as proposed in U.S. Pat. No. 5,702,579 by Veits. Conductivity is the ability of a material to conduct electric current. Instruments that measure conductivity may employ an anode and a cathode as two plates of a circuit having an electrical potential applied across the plates, which may be via a sine wave voltage, and the current that passes through the solution and completes the circuit is measured. FIG. 2 is a depiction of an aspect of an exemplary instrument for measuring conductivity employing an anode and a cathode as two plates. The graphs shown in FIGS. 3A and 3B illustrate the relationship between conductivity and ion concentration for two common solutions. Notice that the graph is linear for a sodium chloride solution, but nonlinear for highly concentrated sulfuric acid. Ionic interactions can alter the linear relationship between conductivity and concentration in some highly concentrated solutions. FIG. 3A is a graphical depiction of the relationship of the ion concentration of sodium chloride in solution with electrical conductivity. FIG. 3B is a graphical depiction of the relationship of the ion concentration of sulfuric acid in solution with electrical conductivity. FIG. 4 is a graphical depiction of an in-line placement of a circuit element of an electrical conductivity measuring device.
Most conductivity meters have a two-electrode cell, as in FIG. 2, available in either dip or flow-through styles. The electrode surface is usually platinum, titanium, gold-plated nickel, or graphite. Four-electrode cells use a reference voltage to compensate for any polarization or fouling of the electrode plates. The reference voltage ensures that measurements indicate actual conductivity independent of electrode condition, resulting in higher accuracy for measuring over wide ranges. Conductivity (G), is the inverse of resistivity (R), i.e., G=1/R, and conductivity may be determined from the voltage and current values according to Ohm's law (V=IR) where since the charge on the ions in solution facilitates the conductance of electrical currents, the conductivity of a solution may be proportional to the solution's ion concentration.
Conductivity may be measured via sensor elements directly immersed in the solution as in FIG. 2. Alternatively, one or more of the sensor elements may be encased in protective material before immersion or placed outside of the direct flow path. FIG. 4 is a graphical depiction of an in-line placement of a circuit element of an electrical conductivity measuring device. Toroidal coils, having an annular presentation to a flow line, may be positioned or encased so as not to be in contact with the solution. Toroidal coils may be either encased in a polymeric material or are external to a flow through cell. A toroidal conductivity measurement is made by passing an AC current through a toroidal drive coil, which induces a current in the electrolyte solution. This induced solution current, in turn, induces a current in a second toroidal coil, called the pick-up toroid. The amount of current induced in the pick-up toroid is proportional to the solution conductivity.